1. Field of the Invention
The present invention relates to AMOLED displaying techniques, and in particular to an AMOLED display device and method for precisely compensating aging thereof.
2. The Related Arts
AMOLED (Active Matrix Organic Light-Emitting Diode) panel shows various advantages, such as high response speed, high contrast, ad wide view angle, as compared to a traditional liquid crystal panel.
AMOLED is a type of OLED (Organic Light-Emitting Diode). Light emission of OLED is realized through electroluminescence caused by injection and recombination of carriers of organic semiconductor material and light emissive material under an electric field. The principle of emitting light of OLED is that an ITO (Indium Tin oxide) transparent electrode and a metal electrode are used as anode and cathode of a device and, being driven by an electric field, electrons and electronic holes are respectively injected into electron and hole transportation layers from cathode and anode. The electrons and the holes migrate through the electron and hole transportation layers to a light emissive layer and meet each other in the light emissive layer to form excitons that excite light-emissive molecules, while the later gives off visible light due to radiative relaxation. The radiating light can be observed at one side of the ITO. The metal electrode layer also serves as a reflection layer.
Compared to a TFT-LCD (Thin-Film Transistor Liquid Crystal Display), an OLED has various advantages, such as being thinner and lighter, active emission of light (requiring no backlighting), being free of view angle issue, high sharpness, high brightness, fast response, low power consumption, wide range of operation temperature, excellent resistance to vibration, low cost, being capable of flexible displaying, of which some are not possible with TFT liquid crystal panel.
However, the AMOLED display uses electrical current flowing through organic material film to give off light. Different organic materials give off light of different colors. With the use of an AMOLED display, the organic material contained in the device may get aging and the light emission efficiency becomes low thereby shortening the lifespan of the display. Different organic materials may age with different speeds thereby leading to different extents of color aging. Further, white field of a display may vary with the use of the display. Further, each pixel may get aging in different speed from those of the other pixels, leading inhomogeneous displaying. Thus, an AMOLED display device must have certain compensation measures to maintain the characteristics thereof.
Referring to FIG. 1, a schematic view is given to illustrate the structure of a conventional AMOLED display device that includes aging compensation function. The AMOLED display device comprises a display panel 11, which comprises AMOLED based pixels; a data processor 12, which processes image data from a video source and compensation data from a compensation unit 16 and outputs compensated image data; a timing controller (TCON) 13, which generates a timing control signal according to the output from the data processor 12; a driver 14, which drives the display panel 11 according to the timing control signal; a display measurement module 15, which measures homogeneity of displaying of the display panel 11; a compensation unit 16, which generates and transmits compensation data according to the measurement result from the display measurement module 15 to the data processor 12. For details of achieving the functions of the compensation unit 16 and the display measurement module 15, reference is made to disclosures of Chinese Patent Application No. CN101194300A and Chinese Patent No. CN1886774B.
Image data of 8-bit color depth from a video source are transmitted to the data processor 12 and the image data of 8-bit color depth processed by the data processor 12 are transmitted to the timing controller 13. The primary function of the timing controller 13 is providing necessary timing control signals to a source driver and a gate driver. The timing sequence of the timing controller 13 is determined by the data structure and displaying mode of the image data. The timing control signal that contains the 8-bit color depth information is transmitted to the driver 14. The driver 14 is generally composed of the source driver and the gate driver. The driver 14 drives the display panel 11 according to the 8-bit color depth and converts it into an electrical current to drive OLEDs of the display panel 11 to give off visible light. Since the aging phenomenon of the AMOLED display device varies with time, the displayed result required by the image data will be different from the actual result of displaying of the display panel 11, leading to inhomogeneous displaying. Consequently, aging compensation may be carried out on the OLEDs. The display measurement module 15 is used to measure the displaying homogeneity of the display panel 11 and then the compensation unit 16 generates 8-bit color depth compensation data. The compensation data are transmitted to the data processor 12 to be processed and mixed in combination with the image data in order to effect compensation of the image data. The compensated 8-bit color depth image data are then transmitted to the timing controller 13 and then applied to the driver 14 to drive the display panel.
However, according to the aging compensation carried out with the conventional ways, the compensation that an integrated circuit (IC) of the driver must be done by at least one level and after conversion into electrical current, the difference of current between two adjacent levels is significant, making it not possible to make precise compensation. A specific condition can be seen from FIG. 2, which illustrates Ids-Vgs characteristic curve of the integrated circuit that drives OLEDs for Vds being equal to −10V and −0.1V. It can be found from FIG. 2 that although the values of Vds may be close, yet the values of Ids are greatly different.
On the other hand, the result of displaying of liquid crystal is determined by the effective voltage applied to the liquid crystal. The gray scale (color) of liquid crystal can be achieved in two ways, namely PWM (Pulse Width Modulation) and FRC (Frame Rate Control). The PWM way divides a scanning time period into a plurality of time slices. For example, for 256 level grey scaling, the division is made for 256 time slices. The number of time slices that are applied with a drive voltage is determined by the grey level to be achieved.
Referring to FIG. 3, a schematic view is given to illustrate the principle of the conventional FRC technique. The FRC technique is a technique that controls frame rate to visually increase the number of colors by applying the phenomenon of persistence of vision of human eyes. FRC uses time average method to make human eyes perceiving different levels of brightness. FRC is similar to PWM but with time slice replaced by frame. As shown in FIG. 3, four time slices correspond respectively to the first frame, the second frame, the third frame, and the fourth frame. For liquid crystal, being of a drive voltage means displaying black. Different visual gray levels can be observed with the persistence of vision based on an arrangement of four successive time slices to which the frames correspond being of driving voltages or not. The way of expressing colors with liquid crystal is similar to that for achieving grey scale and is actually modulation of the grey scales for three primary colors of red, green, and blue.